The present invention relates to a charged particle beam microprobe apparatus for forming the scanned image of a sample by scanning the sample with an electron beam or an ion beam, and more particularly to a charged particle beam microprobe apparatus suitable for extracting three-dimensional information as to the configuration of a particle beam irradiated surface of a sample from an image signal of the surface.
In recent years, an apparatus has been put to practical use which can measure small dimensions on an electron beam scanned surface from the scanned image or image signal thereof, by utilizing the high resolving power of a scanning electron microscope.
The contrast of a scanned image is caused by the fact that the yields of secondary or backscattered electrons from a surface varies with the material forming the surface and the angle of slope thereof. Accordingly, in order to measure the dimensions of the surface, it is necessary to relate bright and dark parts of the scanned image of the surface with the three-dimensional configuration thereof. FIGS. 1A to 1C show an example of a cross-sectional surface structure and scanned images corresponding thereto. Referring to FIG. 1A which is a cross sectional view of a surface to be measured, the so-called line-and-space pattern is formed by providing a line pattern L on a thin layer S which is formed on a substrate, and hence the to-be-measured surface is made of two kinds of materials. FIG. 1B shows a scanned image for a case when the line pattern L and the thin layer S are made of a photoresist material and SiO.sub.2, respectively, and FIG. 1C shows a scanned image for a case where the line pattern L and the thin layer S are made of Si.sub.3 N.sub.4 and SiO.sub.2, respectively. As can be seen from FIGS. 1A to 1C, in order to measure the line width or space width, it is necessary to know whether bright and dark parts of the scanned image correspond to the line pattern L and thin layer S, respectively, or correspond to the thin layer S and line pattern L, respectively. In a case where the dimensions on a surface having a fine structure are to be measured with high accuracy and moreover are automatically measured without utilizing human judgment based upon visual observation of the scanned image, it is very important to determine the three-dimensional structure of the surface.
There has been proposed a method of determining the shape of the cross section of a sample by integrating a signal of backscattered electrons from a surface, under a condition that the coefficient of backscattered electrons is proportional to the angle of slope of the surface, that is, the incident angle of the primary beam to the surface (Refer to a Japanese patent application specification Laid-open No. 150303/81). However, this method is applicable only for a case where the surface is made of a single material and the angle of slope of each part of the surface lies in an angular range in which the above proportionality holds. In the surface of FIG. 1A, which is made of two materials, however, the proportional constant varies with the materials, and hence different integral constants have to be used for different materials, in the integration of the backscattered electron signal. Thus, the above method encounters problems in that it is difficult to determine a boundary between the two materials where one of the integral constants is to be changed over to the other integral constant, and when at least one of the materials is unknown, a corresponding integral constant cannot be determined.